1. Field of the Invention
Cementitious compositions containing mixtures of magnesium oxide (magnesia) and magnesium salts have been known for some time. Of particular interest are magnesia compositions which find utility in spraying, molding and casting applications.
2. Description of the Prior Art
Magnesium oxide based cements are frequently used as fire retardants, usually spray applied to steel and wood structural beams, as filler materials for sandwich-type, insulating wall panels, as flooring or flooring substrate, and for various cast and molded articles where the properties of high heat resistance coupled with light weight and low cost are desired.
The use of magnesia cements as fire-proofing in place of conventional spray-applied asbestos-containing insulation compositions is of special significance in view of recent prohibition in many locales against use of such compositions because of the carcinogenic nature of asbestos. The requirements of costly, heavy-duty hydraulic pumps and sprayers for applying conventional magnesia compositions to walls and beams has, however, retarded use of magnesia cements as fire proofing. Reduction of viscosity by dilution of the compositions with water provides easily sprayable materials, but creates problems of corrosion of substrates, increased curing times and decreased strength properties of cured articles.
Also of increasing interest is the use of magnesia cements as substitutes for natural and synthetic plastics in molding applications, which plastics are derived directly or indirectly from scarce petroleum feedstocks. Cast or molded articles of magnesia in many uses exhibit properties of heat resistance and strength comparable to many organic plastic molded articles. Conventional magnesia cements, however, especially those having fillers or additives, typically have high viscosities and the resulting lack of "pourability" has precluded uses of the compositions for the casting of articles of intricate or complex shape where good flowability of the composition within a mold is a necessity. Adding water to magnesia compositions to reduce the viscosity of a composition to make it pourable or more easily moldable is unsuitable because the increase in volume of the composition produces intolerable degrees of shrinkage or creates voids in the cured product. Additionally, the highest strength magnesia cements are derived from compositions having magnesium salts in saturated or nearly saturated concentrations in water in the wet, pre-cured state; hence, the addition of water may dilute the matrix-forming components to the extent that a solid, integral product cannot form.
Increased use of magnesia compositions would no doubt result if the highly viscous nature of the compositions could be modified to provide more easily sprayable and moldable compositions. Such increased use would generate significant economic benefits inasmuch as magnesia and magnesium-containing salts constitute a significant and practically inexhaustible portion of the material of the earth's natural resources.